Fast Molecular-Dynamics Simulation for Ferroelectric Thin-Film Capacitors Using a First-Principles Effective Hamiltonian

A newly developed fast molecular-dynamics method is applied to BaTiO3 ferroelectric thin-film capacitors with short-circuited electrodes or under applied voltage. The molecular-dynamics simulations based on a first-principles effective Hamiltonian clarify that dead layers (or passive layers) between ferroelectrics and electrodes markedly affect the properties of capacitors, and predict that the system is unable to hop between a uniformly polarized ferroelectric structure and a striped ferroelectric domain structure at low temperatures. Simulations of hysteresis loops of thin-film capacitors are also performed, and their dependence on film thickness, epitaxial constraints, and electrodes are discussed.Comment: 12 figures, 1 table. Submitted to PRB v2->v3: Major changes are underlined in the manuscript. Added new reference

Electric field effect in superconductor-ferroelectric structures

Electric field effect (the E-effect) in superconductors has been studied since 1960 when Glover and Sherill published their results on a shift of the critical temperature T(sub c) about 0.1 mK in Sn and In thin films under the action Off the field E=300 kV/cm. Stadler was the first to study the effect or spontaneous polarization of ferroelectric substrate on the electric properties of superconductors. He observed that the reversal of polarization of TGS substrate under action of external electric field in Sn-TGS structures induced the T(sub c) shift in Sn about 1.3 mK. Since in this case the effect is determined not by the electric field but by the spontaneous polarization, we may call this effect the P-effect. High-T(sub c) superconductors opened the new possibilities to study the E- and P-effects due to low charge carrier density, as compared to conventional superconductors, and to anomalously small coherence length. Experiments in this field began in many laboratories but a breakthrough was made where a shift in T(sub c) by 50 mK was observed in YBCO thin films. Much higher effects were observed in subsequent studies. The first experiments on the P-effect in high-T(sub c) superconductors were reported elsewhere. In this report we shall give a short description of study on the P-effect in high-T(sub c) superconductors

Phenomenological theory of phase transitions in epitaxial BaxSr(1-x)TiO3 thin films

A phenomenological thermodynamic theory of BaxSr(1-x)TiO3 (BST-x) thin films epitaxially grown on cubic substrates is developed using the Landau-Devonshire approach. The eighth-order thermodynamic potential for BT single crystal and modified fourth-order potential for ST single crystal were used as starting potentials for the end-members of the solid solution with the aim to develop potential of BST-$x$ solid solution valid at high temperatures. Several coefficients of these potentials for BT were changed to obtain reasonable agreement between theory and experimental phase diagram for BST-x (x > 0.2) solid solutions. For low Ba content we constructed the specific phase diagram where five phases converge at the multiphase point (T_N2 = 47 K, x = 0.028) and all transitions are of the second order. The "concentration-misfit strain" phase diagrams for BST-x thin films at room temperature and "temperature-misfit strain" phase diagrams for particular concentrations are constructed and discussed. Near T_N2 coupling between polarization and structural order parameter in the epitaxial film is modified considerably and large number of new phases not present in the bulk materials appear on the phase diagram.Comment: 8 pages 5 figure

Concentration phase diagram of Ba(x)Sr(1-x)TiO3 solid solutions

Method of derivation of phenomenological thermodynamic potential of solid solutions is proposed in which the interaction of the order parameters of constituents is introduced through the account of elastic strain due to misfit of the lattice parameters of the end-members. The validity of the method is demonstrated for Ba(x)Sr(1-x)TiO3 system being a typical example of ferroelectric solid solution. Its phase diagram is determined using experimental data for the coefficients in the phenomenological potentials of SrTiO3 and BaTiO3. In the phase diagram of the Ba(x)Sr(1-x)TiO3 system for small Ba concentration, there are a tricritical point and two multiphase points one of which is associated with up to 6 possible phases.Comment: 8 pages, 3 figure

Abrupt appearance of the domain pattern and fatigue of thin ferroelectric films

We study the domain structure in ferroelectric thin films with a `passive' layer (material with damaged ferroelectric properties) at the interface between the film and electrodes within a continuous medium approximation. An abrupt transition from a monodomain to a polydomain state has been found with the increase of the `passive' layer thickness $d$. The domain width changes very quickly at the transition (exponentially with $d^{-2}$). We have estimated the dielectric response $dP/dE$ (the slope of the hysteresis loop) in the `fatigued' multidomain state and found that it is in agreement with experiment, assuming realistic parameters of the layer. We derive a simple universal relation for the dielectric response, which scales as $1/d$, involving only the properties of the passive layer. This relation qualitatively reproduces the evolution of the hysteresis loop in fatigued samples and it could be tested with controlled experiments. It is expected that the coercive field should increase with decreasing lateral size of the film. We believe that specific properties of the domain structure under bias voltage in ferroelectrics with a passive layer can resolve the long-standing `paradox of the coercive field'.Comment: 5 pages, REVTeX 3.1 with two eps-figures. Minor amendments. To appear in Phys. Rev. Letter

Second harmonic generation on incommensurate structures: The case of multiferroic MnWO4

A comprehensive analysis of optical second harmonic generation (SHG) on an incommensurate (IC) magnetically ordered state is presented using multiferroic MnWO4 as model compound. Two fundamentally different SHG contributions coupling to the primary IC magnetic order or to secondary commensurate projections of the IC state, respectively, are distinguished. Whereas the latter can be described within the formalism of the 122 commensurate magnetic point groups the former involves a breakdown of the conventional macroscopic symmetry analysis because of its sensitivity to the lower symmetry of the local environment in a crystal lattice. Our analysis thus foreshadows the fusion of the hitherto disjunct fields of nonlinear optics and IC order in condensed-matter systems

Easy collective polarization switching in ferroelectrics

The actual mechanism of polarization switching in ferroelectrics remains a puzzle for many decades, since the usually estimated barrier for nucleation and growth is insurmountable ("paradox of the coercive field"). To analyze the mechanisms of the nucleation we consider the exactly solvable case of a ferroelectric film with a "dead" layer at the interface with electrodes. The classical nucleation is easier in this case but still impossible, since the calculated barrier is huge. We have found that the {\em interaction} between the nuclei is, however, long range, hence one has to study an {\em ensemble} of the nuclei. We show that there are the ensembles of small (embryonic) nuclei that grow {\em without the barrier}. We submit that the interaction between nuclei is the key point for solving the paradox.Comment: 5 pages, REVTeX 3.1 with one eps-figure. Corrected discussion of single stripe and cylindrical nuclei, and their interaction. The estimate for equilibrium density of embryonic nuclei is added. To appear in Phys. Rev. Letter

Dielectric properties of PbBO₃ perovskites with mixed-valence substitution in the B position

Dielectric properties of PbBO₃ perovskites with mixed-valence substitution in the B position / V. G. Zalesskiĭ, V. V. LemanovThis paper reports on the synthesis of PbBO3 perovskites with mixed-valence substitution in the B position in which the number of ions n in different valence states occupying oxygen octahedra varies from 2 to 6. The dielectric properties of ceramic samples have been studied at frequencies in the range from 12 Hz to 100 kHz and in the temperature interval 77–450 K. The new compounds have been shown to possess relaxor properties. Original Russian Text © V.G. Zalesskiĭ, V.V. Lemanov, 2010, published in Fizika Tverdogo Tela, 2010, Vol. 52, No. 7, pp. 1365–1369

High Dielectric Permittivity in AFe1/2_{1 / 2}B1/2_{1 / 2}O3_{3} Nonferroelectric Perovskite Ceramics (A - Ba, Sr, Ca; B - Nb, Ta, Sb)

AFe$_{1 / 2}$B$_{1 / 2}$O$_{3}$(A- Ba, Sr, Ca; B-Nb, Ta, Sb) ceramics were synthesized and temperature dependencies of the dielectric permittivity were measured at different frequencies. The experimental data obtained show very high values of the dielectric permittivity in a wide temperature interval that is inherent to so-called high-k materials. The analyses of these data establish a Maxwell-Wagner mechanism as a main source for the phenomenon observed.Comment: 6 pages, 7 figure